Conventionally, polyamide fibers are softer and have better touch feeling compared with polyester, and thus are widely used for clothing. A single yarn composed of a single polymer, such as nylon 6 or nylon 66 which is typical polyamide fiber for clothing, has almost no stretch property therein. Therefore, such a single yarn is imparted with a stretch property by, for example, performing false-twisting, and is used for woven or knit fabrics having stretch properties. However, it has been difficult to obtain a woven or knit fabric that has a sufficiently satisfying stretch property from such single yarns that are false-twisted or the like.
For above reasons, a method for obtaining a woven or knit fabric having a stretch property by using elastic fibers or by using two or more polymers with different properties together to form a composite fiber having latent crimp performance has been proposed (see Patent Document 1), the latent crimp performance developing a crimp by heat treatment such as dyeing.
As the elastic fibers, which are the former, polyurethane-based elastic yarns are often used. However, polyurethane-based elastic yarns are typically covered by threads composed of nylon 6 or nylon 66 for use because polyurethane-based elastic yarns have problems in dyeability and light resistance. This still leaves problems such as increase in the mass of the product, increase in costs for the covering process and costs for the polyurethane-based elastic yarn product, and heaviness of the weight of the fabric.
As another example of stretch fibers, composite fibers in which nylon-based elastomer and polyamide are put together in side-by-side arrangement or in eccentric core-sheath arrangement are proposed (see Patent Document 2). This proposal has a problem that the stretch property is lost in processes such as a purification process and a dyeing process due to the characteristic swelling property of polyamide and thus the products do not have sufficient stretch.
Further, even in the case where such polyamide fibers are excellent in crimp properties and exhibit good stretch in a state of an original yarn or a processed yarn, the woven or knit fabric requires to be processed with a tensile force applied thereto in a wet-heat process in order to keep the quality of the woven or knit fabric. This is because generation of wrinkles particular to polyamide fibers is likely to occur in wet-heat processes such as purification and dyeing processes of the woven or knit fabric and the wrinkles generated in the wet-heat processes are not likely to be removed in a dry-heat process of a heat setting process. As described above, there is a problem that the crimp that the original yarn or processed yarn has cannot be sufficiently developed due to the tensile force applied to the woven or knit fabric in the wet-heat process, and as a result the woven or knit fabric has a poor stretch property.
Patent Document 3 proposes a material of latent crimp polyamide that is good in warp stretch and recovery property in a woven fabric. However, polyamide tends to swell with water included therein, is likely to have wrinkles or embossment generated therein in a process of purification, relaxation, or dyeing, and reduces stretch thereof because polyamide is generally processed while being extended. Although a relatively high stretch can be achieved if the tensile force in the processing is reduced in order to improve stretch, it is difficult to improve the quality in terms of wrinkles or embossment.